The relation to be proved looks pretty simple. However, there is no evident point from whih I can start. How do I start relating quantities that are under root? The cyclic order of the sides on the left side is reminiscent of the cosine rule, but that's just it! Plus the right hand side has the sum of three surds. I don't see a drect arrival at such an expression. So, where do we start from? Is there a relation to be picked up which can then be manipulated to reach both the sides? Or some geomterical constraint that we need to be thinking about?